Lead free soldering of Engeneering Thermoplastics

The two EU Directives on electrical and electronic equipment – WEEE Directive 2002/96/EC (Waste Electrical Electronic Equipment) and RoHS Directive 2002/95/EC (Restriction of Hazardous Substances), both of 13 February 2003 – have the key objectives to save natural resources and to minimise dangerous substances in the environment. The RoHS Directive  must be implemented uniformly throughout the European Union on the first of July 2006.

This Directive prohibits, among other things, the use of lead in solder alloys and electronic components. The main consequence of using lead-free solder alloys is a higher soldering temperature. Thermoplasts with low heat resistance can generate problems. In the case of surface-mounted devices (SMD), in particular, the base materials used have to be adapted to these changed temperature conditions. The different soldering methods and suitable Ticona materials are discussed below.

Content:

1. Soldering methods
2. Ticona materials

1. Soldering methods

The most common soldering methods are reflow and wave soldering (Fig. 1). Laser and manual soldering are also used in industry but these methods are of lesser importance in the mass production of electronic components. While in wave soldering, a wave of molten solder alloy is brought into contact with the soldering points, in reflow soldering the solder paste previously applied to the soldering point is heated according to a defined temperature profile by an external energy source in a reflow oven.

In the reflow oven, the paste melts and joins the component contacts to the conductor track. Depending on the energy source, a distinction is made between infrared, convection and vapour-phase reflow soldering. In industry, convection reflow soldering is mainly used today but vapour phase soldering is gaining importance.

Because of the higher melting points of the solder alloys used for lead-free soldering, the maximum process temperatures are about 40 K higher than those in the conventional soldering process. The maximum process temperatures in the lead-free reflow soldering process range from about 250 to 265°C. The component to be soldered is exposed to this temperature for about 10 – 15 s. A typical lead-free reflow process curve is shown in Fig. 2.

2. Ticona materials

Materials used in lead-free soldering processes must meet the following requirements:

• High thermal stability
• Resistance to fluxes and cleaning agents
• High heat deflection temperature:
  HDT/A > 220°C for unstressed components
  HDT/A > 260°C for stressed components   
• High dimensional stability:
• Zero or only very low water absorption
• Expansion coefficient in the same range as that of the printed circuit board

The heat resistance requirements for the insulating material in lead-free soldering are very high. In surface-mounted devices (SMD), particularly, the carrier materials must be capable of withstanding the increased temperatures. This is an ideal application for LCP and PPS. Both liquid crystalline polymer and polyphenylene sulphide offer excellent property profiles. The two materials have a high heat deflection temperature (HDT-A > 260° for stressed components). In addition, they feature low expansion coefficients in the area of the circuit board and excellent dimensional stability. Vectra LCP and Fortron PPS absorb little or no water and are resistant to fluxes and cleaning agents.

The following Vectra LCP and Fortron PPS grades have a very high heat deflection temperature and a high melting point and are therefore suitable for lead-free soldering:

Grade                                     HDT/A                        Melting point DSC

Fortron 1130L4                    265 °C                       280 up to 285 °C
Fortron 1140L4                    270 °C                       280 up to 285 °C
Fortron 6165A4                    270 °C                       280 up to 285 °C
Vectra E130i                         276 °C                             335 °C
Vectra S135                          340 °C                             355 °C
Vectra S471                          323 °C                             355 °C
Vectra E471i                         270 °C                             335 °C
Vectra E820i Pd                   220 °C                             335 °C
Vectra E820i LDS                220 °C                             335 °C

Further information:

The association ZVEI (Zentralverband Elektrotechnik- und Elektronikindustrie) has put together a brochure on lead-free soldinger. To download this brochure, please click here.

For more information please visit their website.